PERIODIC MOTION 315 Eliminating K and solving for g we get (1) Reversible pendulums which are made for the purpose of determining g are so constructed that the two periods are very nearly equal. Therefore we can write p' = p + 5p? and obtain g jyg-pg g~ w D' (P + spy-- DP* D'2 - D2 = 47T2 P2 (Df -D) + 2 PD'SP + £>' (5P)2 25 +D/ r/^NO is neglected] P2 D'-D P 1 2D' D'-D (2) The approximate expression which is given in equation (2) is better adapted for computing the value of g from experimental data than the more exact expression given in equation (1). This is due to the fact that (Df — D), which cannot be determined with a high degree of accuracy, enters into equation (1) as a factor, while it appears only in the correction term of equation (2). 245. Bifilar Pendulum. — A rigid body which is suspended by means of two parallel strings, as shown in Fig. 142, is called a bifilar pendulum. rf \ -D-«i X FIG. 141. When the body is given an angular displacement about a vertical axis through See Appendix Ai.